Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers, typically sulfur curable, diene-based elastomers. The tire rubber, including its tread portion, is typically reinforced with carbon black reinforcing filler.
For the purposes of this presentation, a tire is viewed as being composed of a circumferential tread and supporting carcass therefor. The carcass is viewed as being composed of relatively conventional elements which include but are not limited to sidewalls, beads, innerliner and the supporting carcass plies, including fabric reinforced plies. A shoulder region of a tire is considered as being a portion of the tire where its sidewall meets its tread. It is not normally a sharp line of demarkation and its actual position may vary somewhat from tire to tire. The bead portion of the carcass is typically composed of a relatively inextensible bundle of wires which is encased in carbon black reinforced rubber and is designed to contact a metal rim on which the tire itself is mounted to form a tire/rim, or tire/wheel, assembly which itself conventionally adapted to be mounted on a vehicle. The rim is typically steel or aluminum, or alloy thereof, and is thus electrically conductive since the metal is considered as having a very low resistance to flow of electricity. The term metal as used herein for the metal rim is intended to be electrically conductive metals such as the aforesaid steel and aluminum rims as would be understood by those having skill in such art.
It is acknowledged that, in some tire constructions, carbon black reinforced rubber components such as for example chippers and chaffers may be positioned in the bead area of the tire construction to assist in cushioning the bead component against the metal rim. In the context of this description, a reference to the aforesaid bead component of the tire carcass is intended to include such other rubber components unless otherwise indicated.
In practice, air pressure is applied to the cavity enveloped by the metal rim and the pneumatic tire carcass.
Such construction elements, or components, of a pneumatic tire and tire carcass, as well as such tire/wheel or tire/rim assembly, are well known to those familiar with such tire art.
Rubber by itself is generally considered as being a substantial electrical insulator or, in other words, a rather poor conductor of electricity.
A carbon black reinforced rubber vehicular tire, while still providing a degree of resistance to flow of electricity, has a considerably higher electrical conductivity, or lower resistance to flow of electricity, than rubber without the carbon black reinforcement.
It is considered herein that a continuous electrical dissipation path is created between the electrically conductive metal rim of a tire/wheel (tire/rim) assembly to the to the ground via the carbon black reinforced rubber of the tire, including its ground-contacting tread component, for such a tire/rim assembly mounted on a vehicle which is intended to travel over the ground.
In this manner, it is considered herein that potential electrical energy which may be created by components of or within a moving vehicle as its tires, or tire/wheel assemblies, are rotating and traveling over the ground, is dissipated from the rim of a tire/rim assembly to the ground via the carbon black reinforced rubber path of the tire carcass and tread, which is understood herein to be primarily the outer rubber surface of the tire.
Thus, in one aspect, it is considered herein that the carbon black reinforced rubber of the tire carcass and associated tread normally provide a continuous path to dissipate electrical energy and thereby retard or eliminate static electrical charge from building up and/or accumulating under dynamic conditions of a rotating tire on a vehicle traveling across the ground.
While most carbon blacks are electrically conductive to some degree, some carbon blacks are more electrically conductive than others and are often referred as being electrically conductive carbon blacks. It is understood or believed that such electrically conductive carbon blacks are sometimes used with various industrial products where electrical conductivity is a factor in their utility. However, insofar as it is known, such electrically conductive carbon blacks are not used for commercially manufactured carbon black-reinforced rubber tire treads primarily because such classified electrically conductive carbon blacks are not normally considered the best rubber reinforcing carbon blacks for rubber tire treads.
For the purposes of this invention, electrically conductive carbon blacks are those which, when blended in amount of 50 phr with a diene-based elastomer such as, for example emulsion polymerization prepared styrene/butadiene copolymer, and the resulting mixture sulfur cured, provide a composition demonstrating a surface electrical resistance of 10,000 ohms or less according to ASTM D257-66, Mod G, as hereinafter described.
In one aspect, carbon black reinforced rubber tires may sometimes be prepared which have outer rubber treads designed to be ground contacting which are quantitatively reinforced with silica and thus contain only minimal amounts such as for example 10 phr or less of carbon black.
In such silica reinforced tire tread construction, although the various other rubber components of the tire, namely the aforesaid tire carcass, are quantitatively reinforced with carbon black and thus have a degree of electrical conductivity, the silica reinforced tread itself has a substantially lower electrical conductivity, or in other words a substantially higher electrical resistance and thus create a degree of electrical insulating effect between the tire carcass and ground. Such a tire construction has a substantially less tendency to dissipate static electricity from the tire to the ground, and particularly from the metal rim of a tire/rim assembly to the outer surface of the tire tread and thence to the ground, which may be generated by a dynamic condition of the rotation of the tire on a moving vehicle. Accordingly, a potential for static electricity to build up, or increase, is considered to be higher for such a tire construction than for a similar tire with a carbon black reinforced tread.
Therefore, it is desirable to provide a designed path for electrical leakage or static electricity dissipation for such a tire having a quantitative silica reinforced rubber tread and minimal, if any, carbon black reinforcement.
It acknowledged that it might be thought of to apply a coating of carbon black-containing rubber composition over the outer surface of the silica reinforced rubber tread to facilitate an electrical leakage or dissipation path connecting the tire's carbon black reinforced rubber sidewall region to the ground as the tire rotates on the vehicle. Indeed, elastomer/carbon black coatings, which may be applied as either water-based or organic solvent-based compositions and which are sometimes called pre-cure paints, are often applied to various surfaces of green, or unvulcanized, tire constructions before the tire is vulcanized. A purpose for such pre-cure paints is to reduce friction between the tire and its associated vulcanization mold and increase air bleed between the tire and mold during the vulcanization operation. For example, see U.S. Pat. Nos. 4,857,397 and 4,329,265.
However, it is recognized that a thin outer rubber coating, if applied to a tire tread surface, will relatively quickly wear away as the tire is used, leaving the coating on surfaces within the grooves in a tire tread which has a lug/groove configuration. Thereby, it is considered herein that only a very small portion of the coating on the walls of the tire tread grooves is actually available to be directly presented to, or contact, the ground to facilitate electrical dissipation from the tire to the ground.
It is, therefore, considered herein that in order to be practical, such a thin carbon black-containing rubber coating must be very highly electrically conductive for purposes of dissipating electrical charge from the walls of silica reinforced rubber tire tread grooves to the ground without relying upon a coating on the outer surface of the tire lugs itself.
While the dissipation of generated electrical energy may not be completely understood, it is believed that the electricity is transmitted primarily on the surface of the carbon black reinforced rubber surface of the tire carcass and aforesaid coating from the steel rim to the ground.
It is highly desirable for such a coating to be applied as a water based, film forming carbon black containing rubber composition. By desiring that the film coating be film forming in nature as it is applied to the rubber surface, it is envisioned that it forms a substantially continuous film on the tread surface and ultimately the surfaces of the grooves therein. It is also desirable that the coating adequately adheres to the tire tread surface, ultimately its groove walls.
However, it is a particular challenge to formulate a water based rubber composition composed of a blend of rubber latex and water dispersion of electrically conductive carbon black which will, after coating onto an unvulcanized rubber composition, dry within an acceptable period time to be practical in a commercial manufacturing circumstance. Commercial electrically conductive carbon black water dispersions are understood to be conventionally supplied in concentrations on an order of 20 to 25 weight percent of carbon black. Most synthetic rubber latices are commercially provided, for example, in elastomer concentrations on an order of about 40-43 weight percent for polybutadiene rubber and about 40 to about 50 weight percent for styrene/butadiene copolymers elastomers. Natural rubber might be provided as a latex with about 65 to 70 percent rubber. It is considered herein that a blending of such aforesaid relatively dilute electrically conductive carbon black dispersion with such aforesaid synthetic latices would be expected to yield a water mixture that is too dilute and therefore contain too much water to be highly practical for commercial tire manufacturing practices where relatively short drying times for the coating are desired unless the composition is applied to a hot rubber surface and/or external heat is applied to the coating after it is coated onto the rubber surface.
In one aspect, for a tire tread conventionally configured with a combination of lugs and grooves, it is desired that the grooves communicate with the carbon black reinforced rubber shoulder of the tire, the area of the tire where the sidewall and tread meet, in order for the coating to communicate with the carbon black reinforced rubber portion of the tire, namely the tire carcass and the tread base in a tread cap/base construction.
In a tire tread cap/base construction which is well known to those having skill in such art, and for the purposes of this invention, it is envisioned that the cap is substantially silica reinforced and its base is substantially carbon black reinforced.
It is, therefore, desirable that the rubber coating (i) contains a quantitative amount of electrically conductive carbon black for dissipation of electrical energy under the aforesaid conditions (ii) is covulcanized with the rubber tire tread in order that it be integral with the tread and the walls of grooves of a tire tread configuration composed of lugs and grooves (iii) be extremely thin so that it does not appreciably increase the volume of the tire and does not appreciably affect the tread properties of the tread and (iv) be applicable as a water based composition with a suitably short drying time in order to be practical in a manufacturing process.
For the rubber coat to contain a quantitative amount of electrically conductive carbon black, it is considered herein that it means that the coating should contain a sufficient amount of the carbon black to suitably dissipate an electrical charge although it is believed that too high of a carbon black concentration in the rubber coat would cause the coat to excessively crack on the rubber surface and thus interfere with a continuous coating film aspect. In practice, it is also considered herein that the thinness of the coating is important to not add to the volume of the tire in its vulcanization mold and to not detract appreciably from the tire's tread properties. Indeed, in one aspect, it is expected that the film will wear off of the outer surface of tire tread lugs during use of the tire.
In order for such a coating to be successfully water based, it is considered herein that the coating composition should be composed of a blend of (i) one or more of aqueous rubber emulsions having a basic pH selected from emulsion polymerization prepared synthetic rubber(s) and natural rubber latex and (ii) a water dispersion of carbon black having a basic pH. It is considered herein that the basic pH would normally be expected for the aforesaid synthetic and natural rubber emulsions and is highly desirable and perhaps necessary for the carbon black dispersion in order that an addition of the carbon black dispersion to the latex not pre-precipitate or cause a coagulation of the rubber from the emulsion.
It is also desired that the water based coating composition wet the surface of the rubber to be coated and form a continuous film thereon upon drying. For such purpose it is understood that one or more wetting agents and perhaps a thickening agent, defoamer, preservative and biocide might be added to the basic coating composition.
As used herein, the terms "quantitatively reinforced with silica", "quantitative silica reinforced rubber" and the like which may be used in conjunction with rubber tire treads refer to such rubber treads which contain silica in a range of about 40 to about 90 phr and, optionally, carbon black reinforcing filler in an amount of up to about 10 phr.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber".